System for making particle measurements

ABSTRACT

A system for in situ particle measurement employing light scattering. Both dry and liquid particles are measurable.

United States Patent Shofner Mar. 19, 1974 [54] SYSTEM FOR MAKINGPARTICLE 3.094.625 6/1963 Hendrick 356/102 MEASUREMENTS 3.499.159 3/1970Carrier et a1. 356/103 3,202,826 8/1965 Greathouse 356/206 [75]Inventor: Frederick M. Shofner, Knoxville,

Tenn. [73] Assignee: Environmental Systems Exam",'er Ronald wlbenCorporation Knoxville Tenn Assistant Exammer-Conrad Clark Attorney,Agent. or FzrmFitch, Even. Tobm & [22] Filed: Mar. 1, 1972 L d ka Appl.No.: 230,757

References Cited UNITED STATES PATENTS 5/1969 Armstrong et a1 356/23/DETECT0RI 16 [5 7 ABSTRACT A system for in situ particle measurementemploying light scattering. Both dry and liquid particles aremeasurable.

7 Claims, 4 Drawing Figures PHOTO OPTICAL/ SYSTEM PAIENIEUIAR 19 19143.797.937

sum 1 hr 4 OPTICAL/ SYSTEM =&

LASER SYSTEM FOR MAKING PARTICLE MEASUREMENTS This invention relates toelectro-optical instrumentation systems, particularly such systems formeasurement of particles in various environments.

The environmental effects of atmospheric particulates are a mostimportant part of the overall air pollution problem. It is desirable tomeasure the fundamentally important particle size distribution andconcentration at the source and throughout the source-to-terraintransport path in order to determine transport properties,meteorological effects, and direct biological impact upon the lifeindigenous to the area in which the particulate is collected.

Evaporative cooling towers are one source of environmental pollution.Such evaporative cooling towers provide an attractive means for wasteheat disposal. The evaporated water is pure and does not constitute apollution problem except perhaps visual pollution in the form of fogplumes. However, cooling towers are known to emit small droplets ofwater, called drift, which contain the minerals of the water circulatingin the cooling tower. Neither the total amount of emission, thedistribution of the particle sizes, nor the quantity of mineraldeposited on the environs adjacent the tower by the drift has beenaccurately known heretofore.

The prior art includes various instruments for measuring relativelysmall (less than microns diameter) particles. Insofar as is known toapplicant, these prior art instruments require a sample of the mediumwithin which the particles are contained to be captured and transferredto a position within the instrumentation where the particle measurementis conducted. Other such instruments require that a portion of theparticlecontaining medium be drawn through the instrument underartificial flow conditions that are often quite different from theambient or natural flow conditions.

Certain of these prior art instruments employ light scatteringtechniques wherein the particle or particles within the sample isimpinged with a beam of light whereupon the light is scattered in alldirections. It is well known that a portion of the light scattered fromsuch particles may be directed to a detector where the detected light isconverted into a measure of the particle size and, collectively, theparticle size distribution. In situ particle size and distributionmeasurements are not known to be possible employing such prior artdevices due to the requirement that the scattering volume (the volumewithin which the particle must be located in order to be seen") islocated internally of the device and the particle must be collected andtransferred into the test device. Notably. particles of about 50 micronsdiameter cannot be transferred into such prior devices due to theirphysical size causing them to drop out or be expelled from the carriermedium during transport.

Certain other prior art devices rely upon optical or electron microscopybut these are time-consuming and therefore not applicable to in situdeterminations of particle size and distribution. None of these generaltypes of prior particle measuring systems provide satisfactory means formeasuring liquid particles due to the tendency of liquid particles toevaporate and change size during transfer to the instrumentation.Moreover, such prior art systems are not capable of measuring particlesin situ within a medium which keeps the particles moving nor are theycapable of on line" monitoring of moving particles.

As a further matter, the known prior art particle measuring systems areof the stationary" type. That is. the particles must be brought to theinstrument. It is desirable that a system be provided which is amenableto being towed or carried as by an airplane or other vehicle through acloud of particles such as a dust or rain cloud.

It is therefore an object of the present invention to provide anelectro-optical system for in situ determination of particle sizes anddistribution. It is a further object of this invention to provideinstrumentation for accomplishing in situ particle size and distributiondeterminations. It is also an object to provide a system of the typedescribed which is amenable to being towed or otherwise moved through aparticle-containing medium while making particle measurements. It isalso an object to provide a system of the type described wherein aparticle-containing medium moves past a stationary measuring system. Itis a further object of this invention to provide a method for analyzingthe drift of a cooling tower. It is a further object of this inventionto provide such a method wherein the drift is analyzed in situ.

Other objects and advantages of the invention will be recognized fromthe following description, including the drawings in which:

FIG. 1 is a representation of a particle measurement system embodyingvarious features of the invention;

FIG. 2 is a representation of a further embodiment of the disclosedsystem;

FIG. 3 is a representation of apparatus employed for measuring drift;and,

FIG. 4 is a representation of a particle measurement system employingdual instrumentation.

Briefly stated, the present electro-optical system comprises a radiation(light) source whose output beam intercepts the projected acceptancegeometry of a photodetector to define a scattering volume disposedbetween the radiation source and detector and externally of theinstrumentation. Radiation from the source impinges upon a particlewithin the external scattering volume and is scattered by the particle.That portion of the scattered radiation which falls within theacceptance geometry of the detector and which is therefore directedtoward the detector is received and converted to an electrical signalwhose magnitude is related to the size of the particle. The magnitudeand quantity of signals are useful in determining the distribution ofparticle sizes and/or the concentration of particles within theparticle-containing medium.

With reference to FIG. 1, the depicted system includes a light source 10whose output beam 12 is caused to intersect the projected acceptancegeometry 14 (comprising a cone in the depicted system) of aphotodetector 16 which is disposed at an angle with respect to the beam12. The output from the photodetector is fed to any one of severalpossible devices 17 adapted to convert the signals to a usable form. Forexample, the output signal may be fed to an oscilloscope, a taperecorder, a digital counter, a pulse height analyzer, or a mini-computerwith or without a readout. Further, the output signal may be amplifiedby an amplifier 19 or otherwise acted upon after it exits thephotodetector to make the signal suitable for a particular purpose suchas transmission through a coaxial cable 21.

It will be understood that the cross sectional geometry of the outputbeam 12 from the laser may be of any of several selected geometries, butpreferably is of circular cross section. Where this output beamintercepts the projected acceptance cone 14 of the detector, there is avolume defined as the scattering volume 18. It is noted that in theFigures. only two dimensions of the scattering volume are depicted butit will be recognized that the laser beam, the acceptance cone, andconsequently the scattering volume usually are each three-dimensional.For present purposes, this scattering volume refers to the volume withinwhich a particle must be located in order for radiation from the laserto impinge on the particle and for the radiation scattered by theparticle to be directed in a direction suitable for entry into thedetector 16.

In the depicted system, the projected acceptance cone 14 of thephotodetector 16 is established through the use of a pair of apertures20 and 22 whose spatial location with respect to each other and to thelaser beam and photodetector, along with the size of their respectiveopenings 24 and 26, establish the size of the projected acceptance coneof the photodetector. As desired, additional optics, such as a narrowband filter 28, may be inserted between the scattering volume and thedetector for effecting desired changes in the radiation passing from thescattering volume to the detector prior to the entry of such radiationinto the detector.

In accordance with recognized technology, the radiation emitted from thelaser and impinging upon one or more particles within the scatteringvolume is scattered by the respective particles in all directions. Aportion of the scattered radiation from each particle is;directed fromthe scattering volume toward the detector along a path which fallswithin the projected acceptance cone of the detector. Such radiationportion passes through the apertures 20 and 22, thence through thenarrow band filter 28 to be received by the photodetector 16. Also inaccordance with known technology, the intensity of the radiationreceived by the photodetector from a particle is a measure of the sizeof the particle. Within the photodetector, the scattered radiationreceived by the photodetector from each particle is converted to anelectrical signal whose magnitude is uniquely related to the intensityof the radiation scattered from the particle and therefore to theparticle properties, e.g., size. Accordingly, evaluation of theelectrical signal developed within the photodetector and fed therefromas an output signal provides a measure of the particle size. Further,analysis of all of the signals from the detector provides informationregarding the size distribution and concentration of the particleswithin the medium.

It is recognized that when employing a pulsed laser, as will bediscussed hereinafter, each time the laser pulses there is a sampling ofa known volume of a particle-containing medium, such volume being equalto the scattering volume of the system. From a knowledge of thisscattering volume and the number of laser pulses, along with theobtained knowledge of the particle size of each particle detected andmeasured during a given number of laser pulses, the total number ofparticles per unit of volume of the particle-containing medium can beobtained as well as a determination of the size distribution of themeasured particles within the particle-containing medium.

In the disclosed system, the scattering volume is located externally ofthe apparatus employed. That is, the scattering volume is not enclosed,but rather is exposed to ambient atmosphere. In accordance with thisfeature of the invention, in making a particle measurement, thescattering volume is caused to be disposed within the medium whichcarries the particles. That is, the particles are measured in situ bybringing the scattering volume to the particles. Such in situmeasurements are not known to have been possible heretofore. By reasonof this feature, in combination with a pulsed radiation source as willbe described hereinafter, it is possible to make dynamic measurements ofparticles by disposing the scattering volume within a stream of movingparticles. This capability makes the present system useful for field useeither as a stationary unit or for attachment to a vehicle or airplanefor making measurements of particles spread over a large area, as in adust cloud, rain cloud or the like.

Importantly, the external scattering volume provides a means formeasuring liquid particles with accuracies equivalent to the accuraciesheretofore possible when measuring dry particles. Specifically, liquidparticles evaporate when being transferred to the internal scatteringvolumes employed in the prior art, thereby becoming smaller so that thesize measurement obtained is less than the actual size of the liquidparticle when collected. In the present system, liquid particles aremeasured in situ so that a measurement of their true size is obtained.

Still further, the external scattering volume of the present system isparticularly useful for measuring larger size (about 50 microns diameterand larger) particles. Withdrawing samples from a medium for transfer toa prior art system where the particles are large presents the problem oftransporting the particles due to their tendency to be expelled out ofthe carrier medium upon change in its direction of flow or the like.Because the present system makes the particle measurement in situ,without effecting any substantial change in the flow of theparticle-containing medium that will alter the condition orconcentration of the particles in the medium, the present system isparticulary useful in measuring large particles. This feature of thedisclosed system also enhances the accuracy of the particle measurementsand makes them more representative of the true conditions.

As noted hereinbefore, it is preferable that the particles pass throughthe scattering volume one at a time, so that each unit of intensitychange registered by the photodetector represents only one particle. Inthe present system, such condition is obtained by adjusting the size ofthe scattering volume to that size which will result in one particle ata time passing through the scattering volume under the existingconditions of particlecontaining medium flow and the concentration ofparticles in the medium. Adjustment of the size of the scattering volumeis accomplished either by changing the cross sectional area or geometryof the beam (optically or mechanically), changing the size of theacceptance geometry of the photodetector (optically or mechanically), bymechanical limit means, or by a combination of these. It will be evidentthat other more complicated means may be employed to change the size ofthe scattering volume. In any event, it is emphasized that any meansemployed to adjust the size of the scattering volume is not to applyforces to the particle-containing medium which effect substantial changein the particlecontaining medium that alters the condition ordistribution of the particles in the medium. But rather, the medium isto be kept unaltered so that the individual particles in the medium willpass undisturbed through the scattering volume.

The source of radiation for impinging on the particle under surviellancemay be of several forms. One particularly suitable radiation source is alaser of the junction diode type emitting radiation having a wavelengthin the infrared region of the color spectrum. One particularly usefulwavelength is 9.040 A. as emitted by a gallium arsenide junction laserdiode. Whereas lasers emitting radiation within the visible range areuseful in certain applications. the infrared radiation is preferredbecause of the ready availability and relatively less cost of suchlasers. The preferred junction diode laser has a peak power of at leastseveral watts. The radiation beam from the laser preferably is initiallypassed through an optical system 30 designed to form a homogenous beamhaving sharp cut-offs. In the ideal embodiment the beam intensity isconstant across the beam and zero outside. Such optical systems areknown in the art. In the present system it is desirable that the beamapproach the ideal beam as nearly as possible so as to obtain enhancedaccuracy in resolution of the particle size and/or distribution. In theabsence of such a beam. the intensity of the beam varies across thewidth (transversely) of the beam and a particle measured in the centerof the beam will be impinged with radiation that is more intense than ifthe particle were impinged when it is near either of the opposite sideedges of the beam as when the particle is entering or leaving thescattering volume.

The radiation emission of the laser employd in the present system ispulsed so that the radiation beam passes to the scattering volume inincrements of time. The rate of pulsing is chosen in conjunction withthe velocity of the particle and the cross sectional dimension of thescattering volume so that there is a single exposure of a given particleduring the residence of the particle within the scattering volume. Asnoted hereinbefore, the present system is adapted to in situmeasurements of particles and particularly suitable for measuringparticles that are moving. In accordance with this feature of thesystem, the laser is pulsed in time increments substantially equal tothe time that a moving particle resides within the scattering volume. Bythis means, no single particle is exposed to the light beam more thanonce during its passage through the scattering volume. Consequently,only one electrical signal per particle is developed by thephotodetector, and each such signal representative of a particle ofinterest is separated from each such other signal representative of aparticle of interest by at least one electrical signal that is notrepresentative of a particle of interest, with the result that theoutput from the photodetector accurately reflects the size of eachexposed particle.

Conventional photomultiplier detectors may be employed in the presentsystem. A preferred detector, however. comprises a photodiode followedby a high current gain, low noise preamplifier. This photodetector isespecially useful when working in the range of between about 7,000 A.and about 12,000 A. inasmuch as the usual photomultiplier detector failsto function satisfactorily in such range which includes infraredradiation.

With reference to FIG. 2, a further embodiment of the disclosed systemincludes a laser 40, spatially separated from a photodetector 42 havingan annular acceptance geometry 44 developed by a convexo-convex lens 46disposed between the photodetector and the laser. In the depictedsystem. the laser beam 45 is passed through an optical system 47 andenters a radiation trap 48 after it has been intercepted by the annularacceptance geometry of the photodetector and before it reaches the lens46. The lens 46 is oriented with its long axis disposed substantiallyperpendiculary to the face 50 of the photodetector so that that portionof the beam radiation scattered by a particle 52 disposed within thescattering volume 54 (cross-hatched in the Figure) of the system andwhich falls within the annular solid angle acceptance geometry of thephotodetector is directed by the lens to the photodetector. In FIG. 2,the lines 56 and 58 are intended to depict the outer limits of theannular acceptance geometry of the photodetector 42. The rays 60 and 62above the beam 45 and the rays 64 and 66 below the beam depict (in twodimension) that portion of the scattered radiation that passes to thedetector 42. It is to be recognized that the annular solid angleacceptance geometry is threedimensional rather than two-dimensional asappears from the Figure. Accordingly, in this embodiment, a greateramount of the scattered radiation is directed to the photodetector sothat an enhanced output signal is obtained from the photodetector. Thesystem depicted in FIG. 2 is not restricted to lens as depicted butrather it will be evident that mirrors. lens of other geometries. orother configurations of optics are suitable to aid in establishing theacceptance geometry of the photodetector.

In accordance with one feature of the embodiment depicted in FIG. 2, thescattering volume is reduced in length (laterally in the Figure) bymechanical flow plates 68 and 70 which function to block from thescattering volume any particles other than the particle to be measured.The effect of these flow plates, in combination with the instrumentationhousing 72 and 74, is to mechanically define, in part, the scatteringvolume 54 of the system. Notably, the flow plates do not substantiallyalter the ambient conditions of the particlecontaining medium such asthe rate of flow or particle concentration.

When using the system depicted in FIG. 2, the flow plates 68 and 70 arealigned substantially parallel to the direction of flow of theparticle-containing medium so that inertial effects cause the movingparticles to move through the scattering volume along a path 76 that isgenerally perpendicular to the scattering volume. Thus, the scatteringvolume is mechanically limited in size to that volume which is suitableto cause the particles to pass through the scattering volume one at atime.

The disclosed system is particularly suitable for measuring liquidparticles for the reason that the present system does not alter eitherthe medium or the particle contained within the medium. The drift(liquid droplets) found in cooling towers has heretofore eludedmeasurement, both as to the size of the liquid particles and as to theirconcentration. FIG. 3 depicts a system suitable for measuring drift froma cooling tower. With reference to the Figure, a laser is positionedadjacent the output flow (rays 102) of a cooling tower 104 so that thebeam 106 of the laser is directed through the tower output. As desired,the beam 106 is optically treated by an optical unit 108 prior to itsentry into the tower output. The beam 106 is intersected at a locationwithin the tower output by the projected acceptance geometry 110(conical in this Figure) of a photodetector 112 to define a scatteringvolume 114 disposed within the tower output. In the depicted system. thephotodetector output is fed to an oscilloscope 116.

By reason of the scattering volume 114 being disposed within the toweroutput and not presenting an obstruction or otherwise altering the flowfrom the tower. liquid particles 118 of drift (size exaggerated forclarity) move through the scattering volume unimpeded and in theirnatural state. From a knowledge of the flow of liquidparticle-containing air from the tower, the size of the scatteringvolume 114 is selected so that one liquid particle at a time passesthrough the scattering volume. The beam 106 impinges on a particle atleast once during its movement through the scattering volume. The beamradiation is scattered by the particle and that portion of the scatteredradiation which falls within the acceptance geometry of thephotodetector passes to the photodetector which develops an electricalsignal that is proportional to the intensity of the received scatteredradiation and representative of the size of the particle. In thedepicted system, the electrical signal from the photodetector is fed toan oscilloscope 116 for observation and/r recording by conventionalmeans.

As noted hereinbefore, through the use of a pulsed laser, there isobtained repetitive sampling of the medium, each sample volume beingequal to the scattering volume. From a knowledge of the number of thesesamples and the number and sizes of the particles measured over the timeof such sampling. the size distribution and/or concentration of liquidparticles (drift) is readily obtained. Such information then becomesusable in designing cooling towers having minimum drift.

One particular problem when measuring relatively large particles in thepresence of relatively high concentrations of smaller particles is thatthe radiation scattered by the many small particles can produce abackground electrical signal at the photodetector. For example, the fogemanating from a cooling tower comprises many particles that are smallerthan the liquid particles of drift. FIG. 4 depicts an embodiment of thedisclosed system wherein two detectors 112 and 124 define two identicalindependent but closely spaced scattering volumes 114 and 120 disposedwithin the medium that contains the large and small particles. As theparticle-containing medium moves through the respective scatteringvolumes 114 and 120, the photodetector associated with each of thescattering volumes develops an electrical signal representative of theradiation scattered from all the particles disposed within suchscattering volume. In the first scattering volume 114, for example,there normally will exist not more than one large particle plus severalsmall particles so that the resultant signal, V from the firstscattering volume is relatively large, it being representative of both alarge and several small particles. Simultaneously, the photodetector 124associated with the second scattering volume 120 develops a signalrepresentative of the scattered radiation emanating from the particleswithin such second scattering volume. The mathematical probability ofthe second scattering volume containing a large particle simultaneouslywith there being also one in the first scattering volume issubstantially less than such probability of a large particle being inthe first scattering volume at the time of the measurement so that thesignal, V obtaining at the second photodetector is representative of thecollected scattered radiation from only small particles, hence V, isless than V This second signal, V is electrically subtracted as by adifferential amplifier 122 from the first signal, V to obtain an outputsignal. V which has subtracted therefrom the background signal arisingfrom the presence of the small particles.

While the present description has included specific examples andembodiments, it will be understood that there is no intent to limit itby such disclosure. but rather, it is intended to cover allmodifications and alternate constructions falling within the spirit andscope of the invention as defined in the appended claims.

What is claimed is:

l. A system for particle measurement employing radiation scatteringwherein there is relative movement of a particle-containing medium andparticles contained therein with reference to the measurement systemcomprising radiation source means emitting radiation in beam form,

photodetector means having a radiation acceptance geometry whoseprojection intercepts said beam of said source means,

a scattering volume disposed between said source means and saidphotodetector means and defined by the interception of said beam by saidprojected acceptance geometry of said photodetector means, saidscattering volume being disposed within a medium containing the particleto be measured,

means for effecting relative movement between the particles to bemeasured and the scattering volume without applying forces to theparticle-containing medium which effect substantial change in theparticle-containing medium that alters the condition or ditribution ofthe particles in the medium, and

means pulsing said light source means at a rate that results inillumination of said scattering volume not more than once during eachtime period equal to the time period required for a particle to traversesaid scattering volume in a direction generally parallel to thedirection of movement of said particlecontaining medium under the thenexisting conditions of particle movement through said scattering volumeand at least once during the interval between the exit of an illuminatedparticle from said scattering volume and the entry of a furtherilluminated particle into said scattering volume, whereby the particleto be measured moves substantially undisturbed through said scatteringvolume and while disposed within said scattering volume is impinged bysaid beam and scatters a portion of said beam toward the photodetectormeans to cause said photodetector means to develop an electrical signalthat is proportional to the intensity of the radiation scattered to saidphotodetector means by said particle and each such output electricalsignal that is representative of a particle of interest within saidscattering volume is separated from each other such signalrepresentative of a particle of interest by an output electrical signalthat is not representative of a particle of interest.

2. The system of claim 1 wherein said radiation source comprises a laser3. The system of claim 1 and including means for adjusting saidscattering volume to a size such that under the conditions of flow ofthe particle-containing medium the particles within the medium movethrough the scattering volume one at a time.

4. The system of claim 1 wherein the particle to be measured is a liquidparticle.

5. The system of claim 1 wherein the major dimension of the crosssection of the particle to be measured is larger than about 50 microns.

6. A method for making in situ measurements of particles moving along apath comprising the steps of projecting a beam of radiation into aparticlecontaining medium,

positioning a photodetector means at a location distant from saidradiation beam,

aligning said photodetector means with respect to said beam whereby theprojected acceptance geometry of said photodetector means interceptssaid beam at a location within the particle-containing medium to definea scattering volume,

effecting relative movement between the particle to be measured and thescattering volume without applying forces to the particle-containingmedium which effect substantial change in the particlecontaining mediumthat alters the condition or distribution of the particles in themedium, admitting said beam of radiation to said scattering volume inpulses timed to illuminate said scattering volume not more than onceeach time period equal to the time period required for a particle totraverse said scattering volume in a direction generally parallel to thedirection of movement of said particle-containing medium under the thenexisting conditions of particle movement through said scattering volumeand at least once during the interval between the exit of an illuminatedparticle from said scattering volume and the entry of a furtherilluminated particle into said scattering volume, whereby the particleto be measured moves substantially undisturbed through said scatteringvolume and while disposed within said scattering volume is impinged bysaid beam and scatters a portion of said beam toward said photodetectormeans to cause said photodetector means to develop an electrical signalthat is proportional to the intensity of the radiation scattered to saidphotodetector means by said particle and each such output electricalsignal that is representative of a particle of interest within saidscattering volume is separated from each other such signalrepresentative of a particle of interest by an output electrical signalthat is not representative of a particle of interest. 7. A system formeasuring relatively large particles employing radiation scatteringwherein there is relative movement of a particle-containing medium withreference to the measurement system and said medium contains large andsmall particles comprising a radiation source means emitting a radiationbeam directed into said particle-containing medium,

a pair of photodetector means each having a radiation acceptancegeometry whose projection intercepts said beam at spaced apart locationswithin said particle-containing medium,

a plurality of scattering volumes disposed between said source means andrespective ones of said photodetector means and defined by theinterception of said beam by the respective projected acceptancegeometries of said plurality of photodetector means,

means for effecting relative movement between said particle-containingmedium and said scattering volumes without applying forces to theparticlecontaining medium which effect substantial change in theparticle-containing medium that alters the condition or distribution ofthe particles in the medium, whereby the particles in said medium movesubstantially undisturbed through said scattering volumes and whiledisposed within said scattering volumes are impinged by said beam andscatter a portion of said beam toward respective ones of saidphotodetector means to cause said plurality of photodetector means todevelop respective electrical signals that are proportional to theintensity of the radiation scattered to each such photodetector means bysaid particles, said signals being developed substantiallysimultaneously by said photodetectors so that the mathematicalprobability of a large particle appearing in one of said scatteringvolumes at the time of measurement is substantially less than themathematical probability of a large particle appearing simultaneously inthe other of the scattering volumes and the signal developed by thatphotodetector whose scattering volume contains a large particle isgreater than the signal developed by the other photodetectors, and

means electrically subtracting the smaller of said signals from thelarger signal.

I. I. F t

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 797,937 D d May 13 1974 Inventor(s) Fredrick M. Shofner It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 5, line 13, change "laser diode" to diode laser Signed and sealedthis 17th day of September 1974,

(SEAL) Attest:

MCCOY M. GIBSON JR. Attesting Officer C. MARSHALL DANN Commissioner ofPatents U5COMM-DC 80376-P69 s u.s. GOVERNMENT PRINTING OFFICE: I9690-366-334,

1. A system for particle measurement employing radiation scatteringwherein there is relative movement of a particlecontaining medium andparticles contained therein with reference to the measurement systemcomprising radiation source means emitting radiation in beam form,photodetector means having a radiation acceptance geometry whoseprojection intercepts said beam of said source means, a scatteringvolume disposed between said source means and said photodetector meansand defined by the interception of said beam by said projectedacceptance geometry of said photodetector means, said scattering volumebeing disposed within a medium containing the particle to be measured,means for effecting relative movement between the particles to bemeasured and the scattering volume without applying forces to theparticle-containing medium which effect substantial change in theparticle-containing medium that alters the condition or ditribution ofthe particles in the medium, and means pulsing said light source meansat a rate that results in illumination of said scattering volume notmore than once during each time period equal to the time period requiredfor a particle to traverse said scattering volume in a directiongenerally parallel to the direction of movement of saidparticle-containing medium under the then existing conditions ofparticle movement through said scattering volume and at least onceduring the interval between the exit of an illuminated particle fromsaid scattering volume and the entry of a further illuminated particleinto said scattering volume, whereby the particle to be measured movessubstantially undisturbed through said scattering volume and whiledisposed within said scattering volume is impinged by said beam andscatters a portion of said beam toward the photodetector means to causesaid photodetector means to develop an electrical signal that isproportional to the intensity of the radiation scattered to saidphotodetector means by said particle and each such output electricalsignal that is representative of a particle of interest within saidscattering volume is separated from each other such signalrepresentative of a particle of interest by an output electrical signalthat is not representative of a particle of interest.
 2. The system ofclaim 1 wherein said radiation source comprises a laser.
 3. The systemof claim 1 and including means for adjusting said scattering volume to asize such that under the conditions of flow of the particle-containingmedium the particles within the medium move through the scatteringvolume one at a time.
 4. The system of claim 1 wherein the particle tobe measured is a liquid particle.
 5. The system of claim 1 wherein themajor dimension of the cross section of the particle to be measured islarger than about 50 microns.
 6. A method for making in situmeasurements of particles moving along a path comprising the steps ofprojecting a beam of radiation into a particle-containing medium,positioning a photodetector means at a location distant from saidradiation beam, aligning said photodetector means with respect to saidbeam whereby the projected acceptance geometry of said photodetectormeans intercepts said beam at a location within the particle-containingmedium to define a scattering volume, effecting relative movementbetween the particle to be measured and the scattering volume withoutapplying forces to the particle-containing medium which effectsubstantial change in the particle-containing medium that alters thecondition or distribution of the particles in the medium, admitting saidbeam of radiation to said scattering volume in pulses timed toilluminate said scattering volume not more than once each time periodequal to the time period required for a particle to traverse saidscattering volume in a direction generally parallel to the direction ofmovement of said particle-containing medium under the then existingconditions of particle movement through said scattering volume and atleast once during the interval between the exit of an illuminatedparticle from said scattering volume and the entry of a furtherilluminated particle into said scattering volume, whereby the particleto be measured moves substantially undisturbed through said scatteringvolume and while disposed within said scattering volume is impinged bysaid beam and scatters a portion of said beam toward said photodetectormeans to cause said photodetector means to develop an electrical signalthat is proportional to the intensity of the radiation scattered to saidphotodetector means by said particle and each such output electricalsignal that is representative of a particle of interest within saidscattering volume is separated from each other such signalrepresentative of a particle of interest by an output electrical signalthat is not representative of a particle of interest.
 7. A system formeasuring relatively large particles employing radiation scatteringwherein there is relative movement of a particle-containing medium withreference to the measurement system and said medium contains large andsmall particles comprising a radiation source means emitting a radiationbeam directed into said particle-containing medium, a pair ofphotodetector means each having a radiation acceptance geometry whoseprojection intercepts said beam at spaced apart locations within saidparticle-containing medium, a plurality of scattering volumes disposedbetween said source means and respective ones of said photodetectormeans and defined by the interception of said beam by the respectiveprojected acceptance geometries of said plurality of photodetectormeans, means for effecting relative movement between saidparticle-containing medium and said scattering volumes without applyingforces to the particle-containing medium which effect substantial changein the particle-containing medium that alters the condition ordistribution of the particles in the medium, whereby the particles insaid medium move substantially undisturbed through said scatteringvolumes and while disposed within said scattering volumes are impingedby said beam and scatter a portion of said beam toward respective onesof said photodetector means to cause said plurality of photodetectormeans to develop respective electrical signals that are proportional tothe intensity of the radiation scattered to each such photodetectormeans by said particles, said signals being developed substantiallysimultaneously by said photodetectors so that the mathematicalprobability of a large particle appearing in one of said scatteringvolumes at the time of measurement is substantially less than themathematical probability of a large particle appearing simultaneously inthe other of the scattering volumes and the signal developed by thatphotodetector whose scattering volume contains a large particle isgreater than the signal developed by the other photodetectors, and meanselectrically subtracting the smaller of said signals from the largersignal.